Cannot enter edit mode or change to orthographic view?
No, I don't think so. What matters is that the cube is Active (usually the last selected, with a yellow outline).

In Blender there are three selection states:


Unselected
Selected
Active


There can only be one active object at a time. The active object defines which object will go into edit mode when you press &#8633; Tab, which object's options are shown in the properties panel, etc.

Selecting an object with A, select by layer, etc. will not change the active object.

To make the cube active, click on it with  RMB.
That's odd.. Are you sure the mouse cursor was over the 3D view?
I suspect this is because your active object was either the camera or the lamp, not the cube.

You can tell which layer the active object is on by looking at the layers in 3D view > Header. The orange dot represents the active object: